Transcript apical meristems

Kingdom Plantae
Plant characteristics
(most are shared with Green Algae)
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Multicellular eukaryotes
Photosynthetic (almost all)
Cellulose cell wall
Chlorophyll a and b and other photosynthetic
pigments – xanthophyll, carotenoids, etc.
• Starch is main storage molecule
• Unique plant feature – embryos protected by
tissues of parent plant - Embryophyta
Viridiplantae
Streptophyta
Plantae
Red algae
Chlorophytes
Charophyceans
Embryophytes
Ancestral alga
Possible Evolutionary Relationships of Plants and Relatives
Adaptations to Life on Land
Most of the major evolutionary developments we see among
plants are the result of adaptations to life on land - the two
major challenges to living on land are:
• to avoid desiccation or drying out
• to support the body
Plants have cellulose cell walls which they share with their green
algae ancestors - this aids in preventing desiccation; also
stiffen the plant to provide some support
An outer covering called cuticle, made of a waxy substance
called cutin - cuticle is impermeable to water
Specialized structures on their leaves (sometimes on stem as
well) called stomata which allow plant to breathe in carbon
dioxide and breath out oxygen
Stomata on underside of a rose leaf
Adaptations to Life on Land
• The second major trend in dealing with life on land
has been the development of a sophisticated vascular
system –
• A system of vessels or ducts (known as xylem)
running through plants allows them to conduct water
from roots to leaves and stems;
• A complementary system (known as phloem) also
moves sugars and carbohydrates from leaves (where
sugars are produced by photosynthesis) to rest of
plant
Adaptations to Life on Land
• Plants grow from particular parts of the plant
body known as apical meristems – these are
localized regions of the root tip and shoot tip
where cells actively divide and grow
Apical
meristem
of shoot
Developing
leaves
Apical meristems of plant shoots
and roots. The light micrographs
are longitudinal sections at the tips
of a shoot and root.
Apical meristem
of root
Shoot
100 µm
Root
Apical Meristems
100 µm
Adaptations to Life on Land
• Land plants have developed multicellular
embryos that are nutritionally dependent upon
an adult plant to grow – because they possess
this multicellular embryo, the land plants are
often referred to as Embryophytes
Pea Seed - Embryo
Alternation of Generations
• Plants exhibit a phenomena known as alternation of
generations - this phenomena is shared with some
green algae as well
• Plants alternate between a diploid generation - the
sporophyte; and a haploid generation - the
gametophyte
• The names refer to the reproductive structures
produced by each generation
• Plant evolution has tended to place more emphasis on
sporophyte over time
Nonvascular Plants are United By:
• A general lack of specialized vascular tissues
• Sporophytes of mosses, liverworts and
hornworts are almost always smaller than
gametophytes, and get food from the
gametophyte
• All nonvascular plants have motile sperm
which must swim through water to reach the
eggs
• They lack cuticle and stomata
Liverwort – Division Hepatophyta
Liverwort – Division Hepatophyta
• 9000 species
• gametophytes either lie flat on ground and
grow at one end or some have leaves, stems
and rhizoids (fleshy structures for attachment
to ground) like mosses
• sporophyte usually attached to gametophyte,
sporophyte usually unstalked and spherical
Hornwort – Division Anthocerophyta
Hornwort – Division Anthocerophyta
• 100 species
• usually have fleshy gametophytes
• sporophytes are long and horn-shaped, stand
up from surface of flat gametophytes
Moss – Division Bryophyta
Moss – Division Bryophyta
• 15,000 species
• gametophytes are almost always leafy with small simple
leaves; the plant itself may be tufted or creeping
• sporophyte are usually yellow or brown at maturity and
have sporangium at their tip - they are borne individually
on gametophytes
• when moss spores germinate they give rise to filaments of
cells called protonema that look like green algae - the
leafy gametophytes grow up from buds on the protonema
• Many mosses are very important economically
Sphagnum Bogs - Canada
Sphagnum
Peat cutting today - Scotland
Cooksonia – 408 MYA – early
vascular plant
Vascular Plant Features
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Efficient and sophisticated vascular systems
Large dominant sporophytes
Specialized leaves, stems and roots
Development of heterosporous plants
Advanced cuticle and stomata
Eventual evolution of seeds
Vascular Elements of Vascular Plants
• Xylem – tissue that transports water and mineral
nutrients from roots to rest of plant – made up of
hard- walled cells called tracheary elements
• Phloem – tissue that transports sugars and
carbohydrates from areas where they are made by
photosynthesis to all other parts of the plant – made
up of soft-walled cells called sieve elements
Growth in Vascular Plants
• Primary growth – growth due to cell division
at tips of stems and roots – at apical
meristems
• Secondary growth – growth due to cell
division in periphery of plant – growth at
lateral meristems
Plant Secondary Growth
Note Vascular Cambium is the Lateral Meristem
Division Pterophyta - Ferns
Division Pterophyta - Ferns
• 12,000 species – largest group of seedless vascular plants
• found throughout the world, but most abundant in the tropics
• range from aquatic ferns less than 1 cm in diameter to tree
ferns up to 24 m tall
• sporophyte is most conspicuous part of fern life cycle
• have motile sperm, must be in areas with abundant water or
moisture to reproduce
• structure of most sporophytes is to have a stem that runs along
the ground or in the soil called a rhizome; from the rhizome
roots extend into soil; leaves stick up into air and are called
fronds; sporangia are found on leaves – usually in clusters
called sori
Structure of a Fern
Polypodium sori
Division Psilophyta - Psilotum – Whisk Fern
Division Psilophyta - Psilotum – Whisk Fern
• only 2 genera, very few species – all tropical
• no true leaves; vascular structure is the same
throughout the plant axis – no differentiation
• gametophytes occur in the soil; produce
antheridia and archegonia
• motile sperm
Division Sphenophyta
Equisetum hyemale
Division Sphenophyta - Horsetails
• 15 species
• characterized by jointed stems and leaves in
whorls around the stem at each joint
• some fossils are tree-sized but all living
species are 1 m or less in height
• found in most of the world
• motile sperm
Division Lycophyta
Club Mosses
Division Lycophyta - Club Mosses
• 1200 living species; many extinct species
including fossils of 30 m tall trees
• club mosses (Lycopodium)
• spike mosses (Selanginella)
• produce male and female gametophytes
• motile sperm
Evolutionary trends in the seed plants
• The ovule becomes modified into a seed – a
fertilized embryo with a food supply and
protective covering or seed coat
• Continued emphasis on the sporophyte
generation with the gametophyte becoming
nutritionally dependent upon the sporophyte
Development of a Gymnosperm Seed
Seed plants diverged into two groups
• Gymnosperms - naked seeds - seed (ovule)
not covered at time of pollination - after
pollination, ovule becomes covered to form
seed - so the pollination is direct
• Angiosperms - vessel seeds - ovule enclosed
by sporophyte tissue (the carpel) at time of
pollination - so pollination occurs by growth of
pollen tube carrying sperm to ovule pollination is indirect
Division Cycadophyta – Sago-Palms
Division Cycadophyta – Sago-Palms
• 130 living species; many more extinct common in age of dinosaurs
• distinct trunk covered with bases of shed
leaves
• functional leaves at top of stem in a cluster
• pollen and seed cones on separate plants (male
and female plants)
• have motile sperm
Divison
Gingkophyta
Gingko
biloba
Division Gingkophyta - Gingko
• 1 living species; many more extinct
• little changed in 80 million years
• characterized by fan-shaped leaves with
branching veins
• separate male and female trees
• have motile sperm
Division Gnetophyta
Ephedra – Mormon Tea
Welwitschia
Division Gnetophyta
• 3 very different genera; 75 species
• very unusual group
• many angiosperm like features such as strobili
similar to flowers, very similar xylem tissue
• immobile sperm
• Ephedra (Mormon tea) is a shrub, Welwitschia
is a bizarre desert plant, Gnetum is genus of
tropical vines and trees
Division Coniferophyta – Conifers
Division Coniferophyta – Conifers
• pines, firs, spruces, junipers, cypresses, hemlocks,
redwoods
• 600 species
• have leaves (needles) which resist desiccation adapted to life in dry or extreme habitats
• most species are evergreen - don't shed leaves
• immobile sperm - fertilization via pollen tube
• dominant plants of many ecosystems
• major source of lumber and pulp products
Division Magnoliophyta – Flowering Plants
Division Magnoliophyta– Flowering Plants
• 275,000 species (at least)
• have flowers
• double fertilization in ovary, produces embryo and
endosperm (unique to angiosperms)
• oldest fossils from 130 MYA, group is probably 150
MY old; first came to dominate about 80 MYA;
modern familes of plants appear about 65 MYA magnolias, beeches, beans
• immobile sperm
Flower
structure
Amborella trichopoda
• Oldest group of
flowering plants is
represented by just one
species – Amborella
trichopoda – found only
in New Caledonia. Has
long tracheid cells in the
xylem but lacks shorter,
fatter vessels typical of
more recent
angiosperms.
Amborella flowers
The Water Lilies
• The water lilies are
another ancient lineage
with ancient features
such as complex flowers
with many repeated
units of parts – lots of
petals, stamens, etc. In
flowering plant
evolution we see a
reduction in flower part
number.
The two main groups of Magnoliophyta
• Class Dicotyledones or
Magnoliopsida - dicots most flowering trees, mints,
beans, sunflowers, roses,
etc. – the Eudicots
• 190,000 species
• 2 cotyledons in the embryo
• leaves with netlike web of
veins
• flower parts usually in
multiples of 4 or 5
• Class Monocotyledones or
Liliopsida - monocots grasses, sedges, lillies,
orchids, onions, irises derived from dicots by
reducing number of
cotyledons
• 85,000 species
• one cotyledon in embryo often retain endosperm
• leaves with parallel veins
• flower parts usually in
multiples of 3
Rosa californica – typical dicot
Columbia Lily – typical monocot
Success of Flowering Plants due to:
• Flowers that promote efficient transfer of
gametes
• Development of fruits (mainly the covering of
the seeds) and many ways to disperse fruits
• Tough leaves with efficient cuticle and stomata
and advanced vascular systems allowed them
to develop and spread during a relatively dry
period in earth history
• Chemicals which discourage herbivory